<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        .box{
            height: 500px;
        }
        .box-left{
            float: left;
            width: 200px;
            height: 300px;
            background-color: #cdcdcd;
        }
        .box-right{
            background-color: red;
            overflow: hidden;
            height: 100%;
        }
    </style>
</head>
<body>
    <div class="box">
        <div class="box-left">1</div>
        <div class="box-right">2</div>
      </div>
</body>
</html>